1. Field of the Invention
The present invention relates to electronic controlled couplings for transmitting torque with a wet-type multiple disc clutch engageable by hydraulic pressure.
2. Description of the Related Art
A technology is known as a power transmission system used for four-wheel drive vehicles in which a coupling mechanism, such as a multiple disc clutch or a viscous coupling, continuously controls a driving force to be distributed to the rear wheels. In particular, four-wheel drive vehicles equipped with an automatic transmission can utilize a hydraulic power source of the automatic transmission; hence, many four-wheel drive vehicles of this, type use a hydraulic wet-type multiple disc clutch.
In contrast, vehicles equipped with a manual transmission do not have their own hydraulic power source; hence, the vehicles may use a coupling that engages a multiple disc clutch by a mechanical driving force. Such a coupling is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2000-346102. In the coupling of this type, an armature of an electromagnet causes a control clutch, which is a metal multiple disc clutch, to be pressed and engaged, and engaging torque of the control clutch is transmitted to a main clutch, which is a wet-type multiple disc clutch, by a cam mechanism.
In the known coupling described above, the control clutch is operated by electrical signals and a pressure load of the wet-type multiple disc clutch is amplified by a mechanical cam mechanism. This causes problems of heat resistance and wear resistance of the clutch and the controllability of the clutch due to a change in the temperature of a coil.
If a hydraulic wet-type multiple disc clutch is used, hydraulic lubrication and cooling considerably solve the above problems. Since, however, a hydraulic power source is needed, the hydraulic wet-type multiple disc clutch of this type is applicable to limited cases. Moreover, a hydraulic circuit, various pressure-regulating valves, and the like are needed for obtaining a clutch operating pressure, thus causing restricted flexibility in designing.